Several aspects underscore the attraction of using anti-idiotypic antibodies (anti-Id or Ab2) as immunizing agents in cancer patients to induce cancer-specific immunity. Ab2 vaccines are safe, highly specific, easy to produce in large quantities, and may induce immunity to tumor antigens to which a state of tolerance often exists. Although the potential of polyclonal Ab2 as an immunotherapeutic agent for cancer patients has been previously demonstrated, the presence in this heterogeneous Ab2 population of molecules with unknown specificities is disadvantageous for considerations of clinical applications of Ab2 immunizations. Therefore, monoclonal Ab2 will be produced against various monoclonal anti-human tumor antibodies (Ab1) selected for their preferential binding reactivity to tumor cells, tumoricidal activity in human tumor-grafted nude mice, and availability of the corresponding tumor antigen in purified form. Of the various Ab2 populations produced, those Ab2 that bind to the combining site of Ab, and thus might bear the internal image of the corresponding tumor antigen will be selected. The potential of selected Ab2 to induce tumor-specific anti-anti-Id (Ab3) across species barriers will be tested by immunizing rats or rabbits with Ab2 to produce Ab3 sharing idiotopes with Ab1 and exhibiting identical binding specificities. The Ab3 will be tested for cytotoxic activity against tumor cells in culture. Since human tumors can only be grown in immunodeficient animals, tumor-protective effects of Ab2 immunizations will be investigated in a murine thymoma model system. Ab2 bearing the thymoma-associated antigen Thy-1.2 will be produced and used in tumor protection experiments in mice. In addition, the hypothesis that the beneficial effects of immunotherapy with monoclonal anti-tumor antibody in cancer patients are mediated through Ab2 will be tested. We will test whether the Ab2 produced in cancer patients during the course of monoclonal antibody (MAb) immunotherapy will induce the formation of Ab3 and/or anti-anti-idiotypic T- or B-cells that share idiotopes with the MAb and exhibit identical binding specificities. These studies will provide a rational basis for the use of Ab2 as immunotherapeutic agents in cancer patients.